In mobile radio systems, a data signal transmitted from a transmitter to a receiver is subject to distortion by the physical transmission channel. Among other effects, distortion may be caused by a so-called multipath propagation, in which a data signal is influenced for example by reflection, scattering or diffraction at various obstacles—such as at mountains or at buildings. Because of the multipath propagation, a plurality of received versions of a transmitted data signal (replicas) is received at an input of the receiver, said versions being temporally shifted with respect to one another and attenuated differently. The received and distorted replicas are superposed at the input of the receiver to form an incoming data signal. The different propagation times of multipath components thus give cause to a temporal dispersion of the received data signal. Consequently, a symbol of the received data signal may interfere with adjacent symbols or there is interference into a symbol from adjacent symbols. This effect is referred to as intersymbol interference (ISI). The transmission quality of the communication system may be considerably reduced by the ISI.
The received data signal has a channel profile in which the signal energy of a symbol peaks around different points in time to form clusters. The clusters have a width caused by adjacent multipath components. In an urban region typical multipath channel delays are 4 μs, e.g. in mountainous areas, however, multiple echoes of the transmission signal may cause delays of up to 20 μs. The diverse scenarios have to be taken into account by the receiver.
In order to raise the transmission data rate of the communication system, parallel logical channels are often used for data transmission. E.g. this is the case in a system according to the Universal Mobile Telecommunication Standard (UMTS) as defined in Release 99 or Release 4 of the same. An other example is a so-called high-speed downlink packet access (HSDPA). In both cases, the number of logical channels is obtained by a number of spreading codes used in parallel, each logical channel being assigned to a specific spreading code. The logical channels are also referred to as code channels. By construction, the spreading codes are orthogonal to one another, so that the code channels do not interfere with one another on a signal propagation path. The ISI gives rise to interference among the parallel code channels upon dispreading, such that the signal to interference ratio (SIR) decreases. The achievable data transmission rate of the communication system consequently decreases as well.
It is known to use a rake receiver architecture in order to increase the transmission quality. The functioning of a rake receiver consists in separately evaluating high-energy received versions of the received data signal in so called rake fingers, i.e. in individual correlation receivers, and in reconstructing the data signal in respect of amplitude and phase so to maximize the received signal energy.
The functioning of the rake receiver is advantageous if there is sufficient timely separation between different signal paths of the data signal, i.e. between different signal energy clusters. This may be true if the spreading factors are chosen to be high, i.e. a high number of chips is chosen for the spreading of an individual data symbol. A rake receiver can be used for a communication system with low spreading factors only if the number of parallel logical channels remains low. E.g. in HDSPA the number of parallel logical channels is not greater than five.